Purification of water by reverse osmosis (RO) is growing in popularity. Specifically, it is used for seawater desalination, potable water production, cooling tower make-up, process water generation, boiler feedwater pretreatment and the like. It offers the advantages of removing both organic and inorganic contaminants to yield water which is up to 98% pure. If ion exchange polishing is used downstream of the RO unit, it also saves money by reducing the frequency of demineralizer regenerations and their associated waste disposal costs.
The RO process consists of using pressure to drive water through a semipermeable membrane. As the pure water (permeate) passes through the salt rejecting layer of the membrane, the remaining water (retentate) is enriched in contaminants. A turbulence promoter, present within the membrane module, helps prevent the formation of large concentration gradients within the unit. Nevertheless, next to the barrier layer, an area of concentration polarization develops, and the concentrations of common scale forming salts such as CaCO.sub.3 and CaSO.sub.4 often exceed their solubility limits and precipitate on the surface. In addition, the presence of insoluble iron fines may also foul the membrane surface. Such scaling and fouling decreases the production of pure water. To regain the product water flow, higher driving pressures are necessary, which result in increased energy costs and potential damage to the membranes.
A significant operating cost factor of a reverse osmosis system is the cost of the membranes themselves. With proper care membranes can last for years before replacement becomes necessary. If the membranes experience fouling by deposits of material on their surface, this may result in increased energy consumption, or membrane failure which would ultimately cause an unscheduled shutdown and significant replacement costs. Most RO systems incorporate some type of pretreatment system consisting of filters to remove suspended matter which minimizes fouling. Also, chemical addition is often required to inhibit scaling. Foulants are softer, non-crystalline deposits, which adhere to the membrane surface and include: colloids, small particles, oil, biological growth, metal oxides, and silica. Mineral scales are hard, dense crystalline precipitates which include, but are not limited to, CaCO.sub.3, CaSO.sub.4, BaSO.sub.4, SrSO.sub.4, CaF.sub.2, and Mg(OH).sub.2.
RO end users frequently employ chemical pretreatments such as the addition of polymeric scale inhibitors/dispersants to inhibit undesirable mineral scaling. In some cases, inorganic inhibitors such as sodium hexametaphosphate (SHMP) are used.
There are many types of scale inhibitors which have been used. For example U.S. Pat. No. 4,563,284 (Amjad), issued Jan. 7, 1986, discloses a method for inhibiting formation and deposition of scale-forming salts by adding thereto an effective threshold amount of a phosphonocarboxylic acid and a telomeric phosphinocarboxylic acid that contains features of both phosphonates and polyacrylates. U.S. Pat. No. 4,762,621 (Masler, III et al.), issued Aug. 9, 1988, discloses a scale inhibitor comprising a copolymer of an acrylic acid and a lower alkyl ester of itaconic acid. U.S. Pat. No. 4,784,774 (Amjad et al.), issued Nov. 15, 1988, discloses a scale inhibitor containing a homopolymer of maleic acid or a copolymer of a monounsaturated monocarboxylic or dicarboxylic acid or salt thereof containing 3 to 5 carbon atoms and a phosphonoalkane carboxylic acid. U.S. Pat. No. 4,952,327 (Amjad et al.), issued Aug. 28, 1990, discloses a scale inhibitor obtained by adding to an aqueous medium 0.5 to 500 ppm of a copolymer containing at least one of each of the following three monomers: (a) monounsaturated carboxylic acids as well as salts and anhydrides thereof such as acrylic acid, methacrylic acid, or maleic acid; (b) acrylamidoalkane sulfonic acids and salts thereof, such as 2-acrylamido-2-methylpropane sulfonic acid (AMPS.RTM., a registered trademark of the Lubrizol Corporation); and (c) styrene sulfonic acid and its salts. U.S. Pat. No. 4,652,377 (Amjad), issued Mar. 24, 1987, discloses a scale inhibitor comprised of a polyacrylic acid, phytic acid, and a phosphonocarboxylic acid containing at least one phosphono group, at least two carboxylic groups, and a hydrocarbon chain of at least two carbon atoms. U.S. Pat. No. 5,000,856 (Chen et al.), issued Mar. 19, 1991, a scale inhibitor which comprises (a) a maleic acid/allyl sulfonic acid copolymer, (b) an acrylic acid/acrylamidomethylpropyl sulfonic acid polymer, and a phosphonate such as hexamethylenediamine tetraphosphonic acid or 2-phosphonobutane-1,2,4 -tricarboxylic acid, ammonium salt.
The present invention is directed to the use of a terpolymer of acrylic acid/acrylamide/sulfonated acrylamide in conjunction with a phosphonated product as scale inhibitor/iron dispersant in reverse osmosis systems. This novel antiscalant/dispersant has proven particularly effective in inhibiting CaSO.sub.4 scale on RO membranes, as well as in dispersing of iron. The present inventors have developed a suitable dosage profile which demonstrated effective minimum dosages of this antiscalant/dispersant when used in RO systems. Also, the pH and temperature of the RO feedstream must be controlled in order to properly inhibit mineral scaling within the system.
The present invention also provides many additional advantages which shall become apparent as described below.